IMPACTS OF URBANIZATION OF THE GREATER CAIRO AREA ON THE GROUNDWATER IN THE UNDERLYING AQUIFER

Transcription

1 Hydrological Processes and Water Management in Urban Areas (Proceedings of the Duisberg Symposium, April 1990). IAHS Publ. no. 198, IMPACTS OF URBANIZATION OF THE GREATER CAIRO AREA ON THE GROUNDWATER IN THE UNDERLYING AQUIFER Mamdouh M.A. Shahin International Institute for Hydraulic and Environmental Engineering, Delft, The Netherlands. ABSTRACT Cairo, like many capital cities in developing countries, is expanding in urbanization because of its growing population. The consumption of water is increasing with time and with urbanization. The demand is supplied partly from the Nile and partly from the aquifer underlying the Greater Cairo area. Only a small part of the township is served by an old municipal sewage system. At present the sewers are running full and under pressure. The leakage from the water supply and sewage pipes accumulates in the low spots and turns them into waste water pools. It also causes the contamination of the groundwater. The increasing sulphate content is a menace to the concrete foundations of buildings. Improvement and expansion of the water supply and sewage networks are underway. Many years will pass before these works are completed. By the year 2000 the demand will reach more than double its present figure. Unless effective measures are taken to keep the population growth under control the future might not be any better than the present. GENERAL DESCRIPTION OF THE AREA The Greater Cairo (G.C.) area is situated between the 29 o 40 and N latitudes and the 31 00' and E longitudes with an area of 1200 km. The Nile runs through this area in a flood plain 9 to 35 km wide. The plain is bounded by hills to the east and west. There is a network of irrigation and drainage canals in the flood plain (Figure la). The hills are intersected by a natural network of Wadis. These almost coincide with the lines of geological faulting. The area is generally dry with an annual rainfall of less than 40 mm. The maximum mean summer temperature is 28 C and the minimum mean winter temperature is 13 C. The humidity varies between 45% and 84% and the daily evaporation between 2.5 and 15 mm. The surface geology, as described by Said (1962), consists of Cretaceous formations (sand and limestones, marl,...) in the west and eocene formations (limestone deposits) in the east. The Oligocène formations (volcanic and basalt outcropping) can be found west of the pyramids plateau and north of the eastern hills. The Miocene formations (sandstone and marl) are close to those of the Oligocène, whereas the Pliocene formations are located southeast of the pyramids area. The rest is covered by Quaternary deposits (sand dunes, alluvium,...). The cross sections in Figure lb show the subsurface geology of the area. The population of the G.C. area has risen from 4.82 million in 1960 to million in 1981, at an 243

2 Mamdouh M.A. Shahin average rate of 3.6% as shown in Figure 2. STATEMENT OF THE PROBLEM The available data for the period indicate that the total supply per capita has risen from 127 1/d to say 240 1/d. The groundwater component alone jumped from about 15 1/d in to /d in , with a peak of 68 1/d around The excessive withdrawal from the wells in the western part has led to a lowering of 3 m in the water table level in some places. In the eastern part it is rising steadily. Nyrop et al. (1976) attributed this to the execution of new housing projects and leakage from the old pipes. The groundwater up to a depth of 40 m is contaminated. The contamination decreases with Figure la. Location map of the Greater Cairo Area. 244

3 Impacts of urbanization of the Greater Cairo Area Figure lb. Lithological sections II and IIII. depth and disappears at 60 m below surface. Uncontrolled abstraction of shallow groundwater might lead to health hazards. A demand of 1900 M m is expected by the year If the present ratio groundwater: total supply remains unchanged, the amount extracted must reach 380 M m by then. This will lead to a further decline in the groundwater level with an increasing risk of causing damage to many buildings, especially the old ones. The city of Cairo has an invaluable heritage of ancient, islamic and Coptic monumental objects. GROUNDWATER Geohydrologic parameters of the aquifer The groundwater component of the total supply is provided by a number of stations, each consisting of a battery of pumped wells, dug in an aquifer not less than 40 m thick. The aquifer geohydrologic parameters such as the permeability k, the transmissivity T, and storage coefficient S, have been determined and their values included in Table 1. The locations listed in this table are shown on Figure 3. The vertical hydraulic conductivity of the top clay cap varies between 0.05 and m/d. These values decrease substantially with depth. 245

4 Mamdouh M.A. Shahin Groundwater flow and recharge Abdel Salam (1984) has prepared monthly maps, of which Figure 3 is an example, to show the contour lines of groundwater level. The flow direction is also shown. This together with the data in Table 1 and others provide us with the necessary figures for the water balance. The inflow and outflow in the longitudinal direction are estimated at 1 and 27 M m /yr, respectively. The annual volumes extracted by the municipal wells are 155 M,m and those by privatelyowned wells are estimated at about 170 M m. Of the 2.4 M m pumped everyday into the water supply network 60% is lost, of which a certain Table 1. Site Mustorod Geohydrological parameters of the Greater Cairo aquifer SI S2 S3 S4 S5 (Azbakiya) (ElMarg) (Zaytoun) (S.elKheima) (W.elArab) (Tabbin) (R.elFarag) k m/d T 2 in /d S Method Analysis of pumping test result laboratory determination Quoted from Parson (1980) Quoted from the Groundwater Research Institute, Egypt (1982) amount recharges the groundwater. The difference between the input to the sewage system and its maximum carrying capacity is about 0.5 M m là. Much of this difference goes to the shallow layers of the aquifer causing their contamination. The vertical recharge to the aquifer has been estimated at 800 M m /yr. Shanin (1983) estimated the net supply to the river from the aquifer at 450 M m /yr. The amount that remains in storage is about 10 M m /yr, producing a general rise in the water level of 3 to 4 cm/yr. The living quarters frequently inundated are shown on the map Figure 3. Groundwater quality Samples of groundwater have been collected from observation and pumped wells, bore holes and canals for chemical and bacteriological analyses. The chemical analysis included the determination of the total dissolved solids (T.D.S.), cations (Ca + +, Mg+ +, Na + +, K + ), anions (HCO ~,CO ",Cf,SO "), ph and SAR. Additionally, the rare elements 246

5 Impacts of urbanization of the Greater Cairo Area (boron, lead, copper, zinc,...) were also determined. Depending on the amount of T.D.S. the water has been classified into three qualities; suitable, < 500 ppm; fair and usable in the absence of better water, 500 < T.D.S. < 1200; and unsuitable, more than 1200 ppm. The approximate outlines of the areas belonging to these classes are shown in Figure 3. The areas suffering the most from poor quality are those where the water table undergoes a perpetual rise. The increase in T.D.S. is accompanied by a rise in the sulphate content, which is detrimental to the foundations of buildings. In a few samples high figures of manganese and zinc have been observed in the wells to the south west of the river. The areas polluted by sewage water suffer from a high nitrate content. The samples used in the chemical analysis and other samples collected from the irrigation and drainage canals were tested bacteriologically. The analysis was carried out to determine the probable number index, MPN, of conforming in each sample. Another test was performed to provide figures for the density of the fecal coliform group in a unit of 100 cm. These figures have been divided into five classes; (0), zero; (1), 120 microbes; (2), microbes; (3), microbes and (4) larger than 2400 microbes. These classes are located on Figure 3. Class (1) is considered lightly polluted, (2) and (3) moderately polluted and class (4) highly polluted. As most of the samples tested belong to groundwater, it is important to have it treated before consumption. The least pollution is observed in the municipal wells and the worst in the drains waters. The leakage caused by the breakage of the sewers themselves and from the joints comes in contact with the shallow groundwater. This contact is manifested by seepage from the trenches and pits in those areas which are not connected to the public system. Since many wells are not properly isolated from the surface water, some of the pumped water originates from the contaminated surface water and causes the contamination of the whole pumped amount. Mathematical modelling The existing water supply and sewage networks originally designed to serve half a million inhabitants are supposed to serve the interests of ten million at present!! It has been decided to model this situation assuming that the rate of loss remains unchanged. Using the Figure 2 Growth of population and water use with time. 247

6 Mamdouh M.A. Shahin Figure 3 Equipiezometric and water quality map of the area. available hydrologie data one can forecast the future situations. A finite elementbased model was developed for that purpose. Another model was developed with the objective of investigating the changes in the mechanical properties of the top soil and aquifer, with emphasis on the settlement process. Due to some inadequacies of the data fed to each model, none of them was believed to fulfill its objectives. Accordingly, it has been decided to confine their extent of applicability to the eastern part of the G.C. area. There, the groundwater is rising continuously in level and deteriorating in quality. A third model to investigate the water quality in the area has been developed. The situation that prevailed in the period has been simulated. The preliminary results point to a further rise of the water level. To prevent this it has been suggested the water supply and sewage networks are renewed, and more pumped, wells installed. The.yield of the principal stations should be increased from 32.4 M m in 1980 to 57.7 Mm in the year The Ministry of Irrigation, Egypt (1982) concluded that the ultimate lowering in the groundwater level must not exceed 1.0 m below the 1980 level. This result has been set by the soil mechanics model so as to bring the settlement of the buildings up to an acceptable limit. Furthermore, the new pumped wells should be located in a way as to limit the differential settlement. REFERENCES Abdel Salam, M., Prevailing characteristics of Greater Cairo aquifer system. M.Sc. Thesis submitted to Cairo University, Egypt, 220 pp. Ministry of Irrigation/Academy of Science and Technology, Egypt., Groundwater investigation in the Greater Cairo area. Final report of the first stage (in Arabic), Cairo, 190 pp (unpublished). Nyrop, R.F., Benderly, B.L., Cover, W.W., Eglin, D.R. and Kirchner, R.A., Area handbook for Egypt, U.S. Government Printing Office, Washington D.C., 454 pp. Said, R., The geology of Egypt. Elsevier, Amsterdam, 377 pp. Shanin, M., Investigation of groundwater to the Ministry of Irrigation and 248

7 Impacts of urbanization of the Greater Cairo Area Academy of Science and Technology, Egypt, 23 pp (unpublished). 249

8 250